The present invention relates to a lubricating structure of an engine for a vehicle comprising a main pump which pressure-feeds lubricating oil collected in a transmission room to lubrication points of the engine, and a scavenging pump which pumps the lubricating oil out of a crankcase and is transferred back to the transmission room.
In the lubricating structure of the engine for vehicle comprising the main pump for pressure-feeding lubricating oil and the scavenging pump for transferring the lubricating oil, the scavenging pump functions to maintain the oil level in the crankcase below a predetermined level (about zero) by pumping up the lubricating oil from the crankcase and, for example, transferring it into a transmission room etc. Conventionally, since the lubricating oil collected on the bottom of the crankcase contains many air bubbles, the capacity of the scavenging pump to pump this up is about 1.5 times of the main pump.
The main pump for pressure-feeding the lubricating oil sucks the lubricating oil collected for example, on the bottom of the transmission room through a strainer, pressurizes the lubricating oil to a predetermined pressure, and feeds the pressurized lubricating oil onto a circumference of a crankshaft and to each of the lubrication points within cylinder head(s), while the main pump also feeds the pressurized lubricating oil to the lubrication points in the transmission room.
Since the main pump is designed to pressure-feed the lubricating oil to substantially all lubrication points as mentioned above, the main pump is required to have a pumping capacity that covers all of the required quantity of the lubricating oil. Especially when pressure-feeding the lubricating oil to a fitting portion between a crank pin and a connecting rod through an oil passage formed in the crankshaft, the main pump is required to generate a relatively high oil pressure. Therefore, the oil passage is so constructed that the above-mentioned oil pressure is not reduced by for example, providing a throttle/nozzle in the oil passage within the transmission room.
(1) With the configuration in which the lubricating oil is supplied to the all lubrication points of the engine only by the main pump, since the main pump must be able to supply a sufficient amount of the lubricating oil to the transmission while maintaining the pressure of the lubricating oil which is pressure-fed to the above-mentioned fitting portions such as the circumference of the crankshaft, a large-sized main pump is needed. The large-sized main pump typically has a large driving force, and thus a power loss thereof is also large.
(2) Moreover, the scavenging pump must also be enlarged in its size corresponding to enlargement of the main pump, and enlargement of the entire engine will not be avoided accordingly.
(3) The lubricating oil pumped up from the bottom of the crankcase by the scavenging pump contains many air bubbles, as mentioned above. Since the lubricating oil is sent directly to the transmission room, the main pump sucks the lubricating oil containing many air bubbles and pressure-feeds it to each of the lubrication points. Therefore, it may come to cause generation of cavitation corrosion and wear on sliding surfaces in the main pump.